Magnetic resonance imaging (MRI) is a clinically important medical imaging modality due to its ability to non-invasively provide highly detailed anatomical images with exquisite soft-tissue contrast. These properties of MRI make it a major tool for image-guided biopsy and image-guided therapy using high intensity focused ultrasound (HIFU), radiofrequency (RF) waves, microwaves, cryotherapy, laser, and radiation.
During the MRI process, a subject is placed in a static magnetic field that remains constant. The magnetic moment of nuclei within the subject becomes aligned with the magnetic field. The subject is also exposed to an oscillating magnetic field having a selected frequency in the RF range of the electromagnetic spectrum. This field causes the nuclei within the subject to resonate. After the RF radiation is switched off, the nuclei continue to resonate. This results in the emission of RF radiation from the resonating nuclei. The emission is detected as an MRI signal. An RF receive coil may also be used to receive the resonating emissions from the subject.
For an RF receive coil with a fixed geometry, the signal-to-noise ratio of magnetic resonance signals from a sample increases approximately linearly with the magnetic field. The closer the RF receive coil is to the sample, the larger the signal-to-noise ratio. Thus, for low fields it is very important that the receive coil be close to the body. The greater the distance between the coil and body, the poorer the MRI image. Therefore, in a typical MR-guided interventional procedure, the subject may be placed in a volume receive coil, or near a surface receive coil laid over the region to be imaged, or a phased array coil may be used.
The placement of the coil is set at the start of the procedure. The coil is made large enough to cover the entire treatment area, so that it can remain stationary throughout the procedure. There are drawbacks to such a setup. With large coils, image quality and speed of MRI will suffer, and accuracy and safety of therapy will be affected. On the other hand, when a smaller coil is set over large organs such as liver, the therapeutic device may need to moved over a large area. The coil will then be an impediment to the movement. The movability of the coil should be considered to design a better MRI system.